#!/usr/bin/perl
use strict;
use warnings;

my $filein ='/path/to/in/file.c';
my $fileout='/path/to/out/file.c';

open(my $fhin,  '<', $filein)  or die "ERROR open $filein ($!)\n";
open(my $fhout, '>', $fileout) or die "ERROR open $fileout ($!)\n";
while(my $line=<$fhin>)
{
  # suchen und ersetzen
  $line=~s/(^\s*#\s*if\(\s*)MY_OLD_CONST\s*==\s*1(\s*\))/$1MY_NEW_CONST == 5$2/;
  print $fhout $line;
}
close($fhout);
close($fhin);